Here is a MWE.
\documentclass{article}
\usepackage{acro}
\DeclareAcronym{lb}{short={LB}, long={large banana}, short-indefinite={an}, long-indefinite={a}}
\begin{document}
\ac{lb}
\Ac{lb}
\acp{lb}
\Acp{lb}
\iac{lb}
\Iac{lb}
\end{document}When I compile this with latexmk -pdf <filename>.tex I get
> latexmk -pdf test.tex
Rc files read:
NONE
Latexmk: This is Latexmk, John Collins, 15 June 2025. Version 4.87.
No existing .aux file, so I'll make a simple one, and require run of *latex.
Latexmk: applying rule 'pdflatex'...
Rule 'pdflatex': Reasons for rerun
Category 'other':
Rerun of 'pdflatex' forced or previously required:
Reason or flag: 'Initial setup'
------------
Run number 1 of rule 'pdflatex'
------------
------------
Running 'pdflatex -recorder "test.tex"'
------------
This is pdfTeX, Version 3.141592653-2.6-1.40.28 (TeX Live 2025) (preloaded format=pdflatex)
restricted \write18 enabled.
entering extended mode
(./test.tex
LaTeX2e <2025-06-01> patch level 1
L3 programming layer <2025-09-02>
(/soft/texlive/2025/texmf-dist/tex/latex/base/article.cls
Document Class: article 2025/01/22 v1.4n Standard LaTeX document class
(/soft/texlive/2025/texmf-dist/tex/latex/base/size10.clo))
(/soft/texlive/2025/texmf-dist/tex/latex/acro/acro.sty
(/soft/texlive/2025/texmf-dist/tex/latex/l3packages/l3keys2e/l3keys2e.sty
(/soft/texlive/2025/texmf-dist/tex/latex/l3kernel/expl3.sty
(/soft/texlive/2025/texmf-dist/tex/latex/l3backend/l3backend-pdftex.def)))
(/soft/texlive/2025/texmf-dist/tex/latex/translations/translations.sty
(/soft/texlive/2025/texmf-dist/tex/latex/etoolbox/etoolbox.sty)
(/soft/texlive/2025/texmf-dist/tex/generic/pdftexcmds/pdftexcmds.sty
(/soft/texlive/2025/texmf-dist/tex/generic/infwarerr/infwarerr.sty)
(/soft/texlive/2025/texmf-dist/tex/generic/iftex/iftex.sty)
(/soft/texlive/2025/texmf-dist/tex/generic/ltxcmds/ltxcmds.sty))))
==> First Aid for acro.sty applied!
(./test.aux)
(/soft/texlive/2025/texmf-dist/tex/latex/translations/translations-basic-dictio
nary-english.trsl)
! Improper alphabetic constant.
<to be read again>
\exp_stop_f:
l.11 \Iac{lb}
?
Without \Iac{lb} in the document all is fine.
This is acro v3.8.
Here is a MWE.
When I compile this with
latexmk -pdf <filename>.texI getWithout
\Iac{lb}in the document all is fine.This is acro v3.8.